The present invention relates to a signal processing stage for a radio frequency tuner. The present invention also relates to a radio frequency tuner incorporating such a signal processing stage and for connection, for example, to a terrestrial aerial, a cable distribution system or a satellite aerial system.
EP 0 720 287 and EP 0 527 029 disclose arrangements for controlling the output power of radio frequency power amplifiers for mobile telephone use. The power amplifier output is attenuated by a controllable attenuator/amplifier and the power at the output of this is detected. The detected power is compared with a reference and the difference is used to control a variable gain stage ahead of the power amplifier.
U.S. Pat. No. 4,441,080 discloses a switched capacitor arrangement providing variable negative feedback around an op amp. There is no indication of possible uses of this arrangement.
According to the first aspect of the invention, there is provided a signal processing stage for a radio frequency tuner, comprising an amplifier, a negative feedback loop, for applying negative feedback to the amplifier, and a gain control circuit responsive to a control signal for varying the amount of negative feedback in accordance with the control signal.
The amplifier may comprise a first transconductance stage. The first transconductance stage may comprise differentially connected first and second amplifying devices.
The negative feedback loop may comprise a second transconductance stage. The second transconductance stage may comprise differentially connected third and fourth amplifying devices.
The third and fourth devices may have output terminals connected to common terminals of the first and second devices, respectively. The first to fourth devices may be of the same conductivity type and the common terminals of the first and second devices may be connected together via a first resistance. The third and fourth devices may have common terminals connected via second and third resistances, respectively, to a first power supply terminal.
The first and second devices may be of a first conductivity type and the third and fourth devices may be of a second conductivity type opposite the first type. The common terminals of the first and second devices may be connected via fourth and fifth resistances, respectively, to a first constant current source. The third and fourth devices may have common terminals connected via sixth and seventh resistances, respectively, to a second constant current source.
The gain control circuit may be arranged to steer an output signal of the amplifier between an output of the signal processing stage and an input of the negative feedback loop in accordance with the control signal. The gain control circuit may comprise fifth and sixth amplifying devices having common terminals connected to an output terminal of the first device and seventh and eighth amplifying devices having common terminals connected to an output terminal of the second device, the fifth and eighth devices having control terminals connected to a first control signal input and the sixth and seventh devices having control terminals connected to a second control signal input. The fifth and eighth devices may have output terminals comprising differential outputs of the signal processing stage and connected via first and second loads, respectively, to a second power supply terminal. The sixth and seventh devices may have output terminals connected to the negative feedback loop and via third and fourth loads respectively, to a or the second power supply terminal.
Each of the amplifying devices may comprise a transistor, such as a bipolar transistor. In this case, the control, common and output terminals comprise the base, emitter and collector terminals of each transistor. More generally, the control terminal of an amplifying device controls the current flow between the common and output terminals and the signal at the output terminal is inverted with respect to the signal at the control terminal. Conductivity type refers to the direction of current flow between the common and output terminals.
According to the second aspect of the invention, there is provided a radio frequency tuner comprising at least one stage according to the first aspect of the invention.
The at least one stage may comprise at least one of a low noise amplifier, a mixer, an intermediate frequency amplifier and a baseband stage.
The tuner may comprise at least one automatic gain control signal generator for generating the control signal for the at least one stage.
It is thus possible to provide an arrangement of improved performance. For example, when used to provide automatic gain control, the greatest level of negative feedback is applied for the highest signal level at the amplifier, so that the intermodulation distortion performance, such as IP3, is substantially improved. For lower signal levels, the negative feedback is reduced but the noise performance can be substantially improved. Because of the lower signal levels, the inter-modulation performance is not required to be as good as for higher signal levels so that an overall improvement in performance for signals of any level can be achieved.